“By no means are we out of the woods here,” said John Callas, the Opportunity project manager at Nasa’s Jet Propulsion Laboratory in Pasadena, California. “This storm is threatening, and we don’t know how long it will last, and we don’t know what the environment will be like once it clears.”

Flight controllers tried late on Tuesday to contact Opportunity, but the rover did not respond. The storm has been growing since the end of May with unprecedented speed.

Nasa launched the twin rovers Opportunity and Spirit in 2003 to study Martian rocks and soil, and they landed in 2004. Spirit has not worked for several years, but Opportunity has kept exploring well past its expected mission lifetime.

Scientists are not as concerned about the newer, nuclear-powered Curiosity rover on the other side of Mars, which is already seeing darkening skies.

(PA Graphics)

Dust storms crop up every so often on Mars, sending dust tens of miles into the atmosphere and turning day into night. Spacecraft orbiting Mars are too high to be affected.

There is no chance of Opportunity being buried or getting a wheel stuck in dust. Even in the worst of storms, only a layer of fine dust is left behind. Managers said the main concern is that dust could temporarily cover its optical instruments.

The rover’s batteries are likely to be so low that only a clock is still working, to wake the spacecraft for periodic power-level checks, according to officials. If the clock also goes offline, then the rover will not know what time it is when it comes back on and could send back signals at any time.

A global map of Mars with a growing dust storm. The blue dot at centre indicates the approximate location of the Opportunity rover (Nasa/JPL-Caltech/MSSS/AP)

This isn’t Opportunity’s first major brush with dust.

In 2007, a massive dust storm kept the rover silent for a few days. It jumped back into action after waking from its deep self-protecting slumber.

This time, the rover’s energy level is believed to be much lower. On the plus side, Martian summer is approaching and should keep temperatures up at night and prevent the batteries and other parts from freezing.

Scientists are eager to learn as much as they can about the dust storm to hone their weather forecasting skills.

Astronauts living on Mars would not want to get caught outside in a fierce dust storm, where winds can reach 70mph — almost hurricane force. The Martian atmosphere is so thin that while the wind can lift dust off the surface, it doesn’t topple a spacecraft.